Protection device against electrostatic discharges

ABSTRACT

An ESD protection device is proposed, encompassing a vertical bipolar transistor that is connected as a diode and has an additional displaced base area. The assemblage according to the present invention has a space-saving configuration and a difference between snapback voltage and breakdown voltage that is decreased as compared to known assemblages.

BACKGROUND OF THE INVENTION

[0001] The invention proceeds from a protective device according to the species defined in the independent claim. DE 197 46 410 has already disclosed a protective device of this kind in which, in the event of electrostatic discharge (ESD), a vertically positioned transistor diode is made conductive by way of a lateral depletion-layer contact effect or “punch-through effect.” In the protective device described therein, however, the breakdown voltage is typically twice as great as the “snapback voltage,” i.e. the minimum voltage that must be present after breakdown between collector and emitter to ensure that the diode remains conductive.

ADVANTAGES OF THE INVENTION

[0002] The protective device according to the present invention having the characterizing features of the independent claim has, in contrast, the advantage of a smaller difference between breakdown voltage and snapback voltage. This is because as a rule, protective structures whose snapback voltage is greater than the operating voltage of the circuit to be protected, and whose breakdown voltage is less than the technology-related dielectric strength of the circuit to be protected, are regarded as reliable. If it is necessary to connect two protective elements in series so that the snapback voltage is greater than the operating voltage, not only the snapback voltage but also the breakdown voltage then doubles. The resulting value for the breakdown voltage can, however, be too high for the circuit. The lateral diode preceding the base-emitter structure advantageously makes it possible to remain below a lower limit for the breakdown voltage, which hitherto was defined by the minimum field oxide extension between the base-emitter structure and the collector connection. In the context of series circuits of protective elements, this decreased breakdown voltage is therefore critical in terms of cost-effective utilization, so that even integrated circuits with high supply voltages can be protected. Combination (series connection) with other ESD protection elements is also improved, so that specific “protection windows” can be established (supply potential of the circuit to be protected as the lower limit for the snapback voltage, maximum permissible voltage of the circuit to be protected as the lower limit for the breakdown voltage).

[0003] The independent claims and the description of features presented make possible advantageous developments of and improvements to the protective devicees described in the main claim. It is particularly advantageous to provide an additional diode trough for the integrated lateral diode, thereby allowing the breakdown voltage of the protective assemblage to be further lowered.

[0004] In addition, the breakdown voltage can be further varied by adjusting the distance between the connector structure and the integrated lateral diode.

[0005] The provision of a recessed electrode makes possible improved contact between the buried layer and the collector electrode, and simultaneously ensures better protection of the circuit and surrounding circuit parts from mutual influences.

[0006] A connector layer of the connector structure can furthermore be selected in such a way that the snapback voltage is suitably dimensioned substantially irrespective of the breakdown voltage.

[0007] Further advantages are evident from the features recited in the further dependent claims and in the description.

DRAWINGS

[0008] Exemplified embodiments of the invention are depicted in the drawings and explained in more detail in the description below. In the drawings:

[0009]FIG. 1 shows an assemblage known from DE 197 46 410;

[0010]FIG. 2 shows a first exemplified embodiment;

[0011]FIG. 3 shows a second exemplified embodiment;

[0012]FIG. 4 is a current/voltage diagram; and

[0013]FIG. 5 shows a further exemplified embodiment.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0014]FIG. 1 is a cross-sectioned side view of a protective device that is positioned on a p-doped semiconductor substrate 30. An n-doped surface area 52 is epitaxially applied onto substrate 30, a highly p-doped buried layer 54 being positioned between the surface area and the substrate. Introduced on surface 10 of surface area 52 is a p-doped trough 50 into which are in turn introduced a highly p-doped area 40 and, immediately adjacent thereto, a highly n-doped area 56, the two being electrically connected to one another via a metallic emitter electrode 44 on the surface. An insulating oxide layer 100 positioned on the surface separates p-trough 50 from a highly n-doped connector layer 42, introduced adjacently thereto into the surface, which can be electrically contacted via a collector electrode 46. Connector layer 42 overlaps with a highly n-doped recessed or “sinker” electrode 540, introduced into surface area 52, which in turn partially overlaps with buried layer 54. The doping of p-trough 50 is typically in the range of 10¹⁷ cm⁻³. The n-doping of surface area 52 is typically in the range of 10¹⁵ cm⁻³, and the doping of the highly n-doped connector layer typically in the range of 10¹⁹ cm⁻³.

[0015] The blocking polarization of the p-n transition between p-trough 50 and surface area 52 is relevant in terms of utilization of an device of this kind for ESD protection. The distance between the p-trough and connector layer 42 is dimensioned such that the depletion zone in surface area 52, which expands with increasing blocking voltage, reaches connector layer 42 before the breakdown voltage between p-trough 50 and surface area 52 is attained. A breakdown thus occurs between trough 50 and connector layer 42 as a result of the depletion-layer contact effect.

[0016]FIG. 2 shows an exemplified embodiment of the protective device according to the present invention. Reference characters identical to those in FIG. 1 designate identical or similar components and are not described again below. A highly p-doped area 500 is introduced into surface area 52 between p-trough 50 and connector structure 540, 46. Area 500 has a higher p-doping than trough 50; the dopant concentration in area 500 is typically in a range of 5×10¹⁹ to 10²⁰ cm⁻³. In addition, highly p-doped area 500 is of more planar configuration than p-trough 50, i.e. it does not protrude as deeply into surface area 52 as does p-trough 50. Area 500 is connected to base area 50 via an electrical connection 510. This connection 510 can be embodied as a metallic conductor on the surface of the semiconductor component, contacting of p-trough 50 being accomplished via a highly p-doped contact area 520 that is introduced into p-trough 50. An insulating oxide layer 530 is positioned between area 500 and p-trough 50 on surface 10 of the component. Connector structure 540, 46 is at a distance y from area 500, y being e.g. between 3 and 8 um.

[0017] The p-n transition between area 500 and surface area 52 results in formation of a lateral integrated diode that serves, when the component is laid out, as a protective device connected in the reverse direction. The negative pole of the circuit to be protected is connected to the emitter electrode, and the positive pole to the collector electrode. Once breakdown of this integrated lateral diode has occurred when a certain voltage value (the breakdown voltage) is exceeded, the base of the vertical transistor is activated in such a way that the vertical transistor effect occurs. The component jumps to the characteristic snapback voltage of the vertical bipolar transistor; the lateral diode breakdown is canceled because the snapback voltage of the vertical transistor diode is lower than the diode breakdown voltage, and the vertical transistor remains activated. Area 500 can also, alternatively, be referred to as a “displaced base area.” Because area 500 is more highly doped than base 50 and/or is more sharply delimited from surface area 52 because of its more planar configuration as compared to base area 50 (i.e. exhibits a greater dopant gradient with respect to surface area 52 than does base area 50), a greater electric field is achieved at a given voltage in the p-n transition of area 500 with surface area 52, and consequently the assemblage breaks down at a lower voltage.

[0018] The breakdown voltage can be lowered further by decreasing distance y. The distance must, however, be selected in very defined fashion in this context, since otherwise highly n-doped area 540 will approach too closely to highly p-doped area 500 and can thus reach the breakdown voltage very easily for utilization at low values. For y=8 um the breakdown voltage is approx. 37 volts, for y=7 um 34 volts, for y=6 um 25 volts, for y=5 um 13 volts, for y=4 um 7 volts, and for 7=3 um approx. 5 volts. The snapback voltage of the assemblage, on the other hand, is determined solely by the vertical transistor and the resistance of the collector contact.

[0019] Sinker electrode 540 is not necessary for the operating principle of the assemblage and can therefore be omitted. In this case it is sufficient to provide a highly n-doped connector layer above which collector electrode 46 is positioned. The resistance of the collector contact is thereby slightly raised and thus the snapback voltage is also increased as compared to an assemblage having a sinker electrode, but the protective function is still ensured. A sinker electrode is, however, useful in order to delimit the protective device with respect to other circuits and thus to suppress parasitic effects and leakage currents into the substrate. Devicees such as those described in FIG. 2 can also be implemented with transposed doping types.

[0020] It is also possible to configure areas 500 and 50 in overlapping fashion. Insulating layer 530 is then omitted.

[0021] In contrast to FIG. 2, FIG. 3 shows a protective device in which displaced base area 500 is embedded into an n-doped trough 550 that is more highly doped than surface area 52. While the surface area, as already explained above, typically has a dopant concentration of 10¹⁵ cm ⁻³, trough 550 is doped at a concentration of approx. 10¹⁶ cm⁻³.

[0022] The presence of n-trough 550 results in a sharper p-n transition between area 500 and the surrounding n-doped area than without this additional trough. This represents an additional contribution to lowering the breakdown voltage of the protective device, since because of the steeper p-n transition, a higher field at the p-n transition is achieved for a given voltage.

[0023]FIG. 4 is a current/voltage diagram (in which I indicates current and U voltage) for the assemblages as shown in FIGS. 1 through 3. Curve A shows the profile for an assemblage as shown in FIG. 1, curve B the profile for an assemblage as shown in FIG. 2, and curve C for an assemblage as shown in FIG. 3.

[0024] For example, if the electrode of the assemblage of FIG. 1 is at ground and collector electrode 46 is at positive potential, curve A then shows that the p-n transition between base trough 50 and surface area 52 breaks down at a voltage of approx. 52 volts as a result of the depletion-layer contact effect at connector layer 42. Only when the voltage has risen thereafter to approx. 56 volts is there a flow of charge carriers around the emitter of the vertical transistor diode such that the p-n transition between emitter and base of the vertical transistor diode becomes conductive and a vertical current flows. In the range between 52 and 56 volts, therefore, at first an exclusively lateral current flows until, as a result of activation of the vertical transistor diode, the total resistance between collector and emitter decreases considerably and the voltage between those two electrodes correspondingly drops to a snapback voltage value in the range between 20 and 30 volts. Because of its higher doping and/or its more planar configuration, the preceding base area 500 causes an earlier breakdown and a correspondingly earlier onset of lateral current flow at a voltage between the collector and emitter electrodes of approx. 36 volts. The voltage range for purely lateral current flow is more distinct; in case B the voltage keeps rising to 44 volts until here again the transistor diode activates and the voltage between its electrodes drops to the snapback voltage value. The much lower breakdown voltage—approx. 44 volts as compared to approx. 56 volts with the assemblage known from the existing art—is clearly evident. Lastly, curve C shows the effect of diode trough 550 on the current/voltage profile: the onset of lateral current flow occurs at an even lower voltage of approx. 24 volts; and the breakdown voltage, i.e. the voltage at which the vertical current flow and thus a decrease in voltage to the snapback voltage begins, is also decreased further and is only approx. 31 volts.

[0025]FIG. 5 shows a variation of the circuit assemblages as shown in FIGS. 2 and 3 which is known per se from the German Patent Application numbered 199 17 155.6 but can advantageously also be applied to protective devicees having displaced base areas. The dashed line at the right edge of the illustration marks the boundary beyond which the assemblage is configured identically to FIG. 2 and FIG. 3. Only in the portion left of the dashed line, in contrast to FIGS. 2 and 3, is a separate connector layer 42 provided, which is highly n-doped and serves for attachment of collector electrode 46. What is essential here is the provision of a distance x between recessed electrode 540 and connector layer 42, thus creating an intermediate area 600 whose dopant concentration represents a local minimum between the recessed electrode and the connector layer.

[0026] As a result of the intermediate area, connector layer 42 is connected only in very high-resistance fashion to the buried layer, since any possible current flow between the connector layer and the buried layer runs through the surface area, which is lightly doped relative to the buried layer and the connector layer. The result of this, however, is that in the event of a breakdown, the transistor diode exhibits a higher snapback voltage as compared to the assemblage of FIGS. 2 and 3. The value of the snapback voltage is thus adjustable by way of the distance x between the connector layer and sinker electrode: it rises as x increases. 

What is claimed is:
 1. A protective device against electrostatic discharges, for an integrated circuit positioned in a semiconductor substrate (30), having a transistor diode (44, 56, 40, 50, 52, 54) positioned in a surface area (52) vertically with respect to the surface (10) of the semiconductor substrate, its base-emitter structure (44, 56, 40, 50) being introduced on the surface and its collector being formed as a buried layer (54), a connector structure (540, 46) for contacting the buried layer (54) being positioned in the surface area (52) laterally offset with respect to the transistor diode, so that when the transistor diode is polarized in the reverse direction, a depletion-layer contact or “punch-through” occurs at the connector structure before a breakdown can occur between the surface area (52) and base-emitter structure (44, 56, 40, 50), wherein between the base-emitter structure (44, 56, 40, 50) and the connector structure (540, 46) there is introduced into the surface area an area (500) whose doping type is selected to be opposite to the doping type of the surface area (52), the area (500) being connected (510) in low-resistance fashion to the base-emitter structure so that the semiconductor transition between the area and the part of the surface area surrounding the area results in formation of a lateral diode for activation of the vertical transistor diode, the dopant concentration in the area (500) being higher than in the base area (50) of the base-emitter structure (44, 56, 40, 50) and/or the area (500) being more planar than the base area (50).
 2. The protective device as recited in claim 1, wherein the area (500) is positioned in a trough (540), oppositely doped and introduced into the surface area, whose dopant concentration is higher than that of the surface area (52).
 3. The protective device as recited in claim 1 or 2, wherein an insulating oxide layer (100) is positioned between the area (500) and the connector structure on the surface of the surface area (52).
 4. The protective device as recited in one of the foregoing claims, wherein a further insulating oxide layer (530) is positioned between the area (500) and the base-emitter structure (44, 56 40, 50).
 5. The protective device as recited in one of the foregoing claims, wherein the distance (y) between the connector structure (540, 46) and the area (500) is dimensioned in accordance with a breakdown voltage of the transistor diode that is to be achieved.
 6. The protective device as recited in one of the foregoing claims, wherein the connector structure has a recessed electrode (sinker electrode 540), introduced into the surface area (52), that overlaps with the buried layer (54) and extends to the surface.
 7. The protective device as recited in one of the foregoing claims, wherein the connector structure has a connector layer (42) to which a collector electrode (46) is attached.
 8. The protective device as recited in claim 7, wherein the connector layer (42) is completely surrounded, on the sides facing away from the surface, by regions of the surface area that have a lower doping than the connector layer (42), so that a current path between the buried layer (54) and the connector layer (42) must always run through the regions.
 9. The protective device as recited in claim 6 and 8, wherein a distance (x) differing from zero between the recessed electrode (540) and the connector layer (42) is dimensioned in accordance with the snapback voltage of the transistor diode that is to be achieved.
 10. The protective device as recited in one of the foregoing claims, wherein the semiconductor substrate (30) is p-doped, the surface area (52) being embodied as an n-doped epitaxial layer.
 11. The protective device as recited in claim 10, wherein the base-emitter structure (44, 56, 40, 50) has a p-doped trough (50) introduced into the surface area (52) and forming the base of the transistor diode.
 12. The protective device as recited in claim 11, wherein a highly p-doped area (40) for contacting the p-trough, and a highly n-doped area (56), are introduced into the p-doped trough, the two areas (40, 56) being in electrical connection with one another via a metallic conductor, in particular via an emitter electrode (44) applied on the surface.
 13. The protective device as recited in one of claims 10 through 12, wherein the buried layer (54) is highly n-doped and is positioned between the substrate (30) and the surface area (52), extending at least beneath the base-emitter structure.
 14. The protective device as recited in claim 7, wherein the buried layer also extends beneath the connector layer (42).
 15. The protective device as recited in claim 7 and 10, wherein the connector layer (42) is highly n-doped.
 16. The protective device as recited in claim 6 and 10, wherein the sinker electrode (540) is highly n-doped.
 17. The protective device as recited in one of claims 10 through 16, characterized by transposed doping types. 